Selective X-ray absorption spectroscopy of self-assembled atom in InAs quantum dot

Transcription

1 Microelectronic Engineering (2003) locate/ mee Selective X-ray absorption spectroscopy of self-assembled atom in InAs quantum dot Masashi Ishii *, Kazunari Ozasa, Yoshinobu Aoyagi a,b, c c a SPring-8, JASRI, Mikaduki-cho, Sayo-gun, Hyogo , Japan b RIKEN (The Institute of Physical and Chemical Research) Harima Institute, Mikaduki-cho, Sayo-gun, Hyogo , Japan c RIKEN (The Institute of Physical and Chemical Research), 2-1 Hirosawa, Wako, Saitama , Japan Abstract To discuss the local structure of an atom in the self-organized InAs quantum dot (QD), we perform a site-selective X-ray absorption fine structure (XAFS) measurement, employing the capacitance XAFS method. Since the X-ray-induced photoemission of confined electrons in a QD via inner-shell absorption can be detected by capacitor, the photon energy dependence of the capacitance provides the XAFS spectrum of the atom in the QD. It is found that when the bias voltage applied to the capacitor aligned the Fermi energy with the quantum level, the site-selectivity is enhanced. The peak energy shift of 1.5 ev is observed in the site-selective XAFS spectrum of an As atom in QD. The molecular orbital calculation indicates that the energy shift originates from the point defect at the boundary between an InAs QD and a GaAs barrier layer Elsevier Science B.V. All rights reserved. Keywords: Capacitance X-ray absorption fine structure (XAFS); Site-selectivity; Self-organized quantum dot; Point defect; Orbital rotation 1. Introduction A nano-structure artificially modifies an electronic state originally intrinsic to the material. Based on this fundamental idea, numerous proposals for nano-device structures with superior electrical or optical performance and various efforts to develop nano-fabrication methods have been demonstrated [1 3]. The quantum dot (QD) is one of the fundamental nano-structures yielding high performance devices by electron confinement, so the fabrication of a well-defined QD has been a crucial research topic for a long time. Recent advancement of crystal growth techniques makes it possible to realize *Corresponding author. Present address: SPring-8, JASRI, Mikaduki-cho, Sayo-gun, Hyogo , Japan. Tel.: ; fax: address: ishiim@spring8.or.jp (M. Ishii) /03/$ see front matter 2003 Elsevier Science B.V. All rights reserved. doi: / S (03)

2 956 M. Ishii et al. / Microelectronic Engineering (2003) coherent QD arrangements. In particular, a self-organization method, in which the QDs with a high area density and high uniformity in size are automatically assembled by a strain-induced growth mechanism, is considered to be a promising crystal growth technique for forming three-dimensional quantum structures [4,5]. Moreover, considering each QD size, it is clear that the local structure at the atomic level determines the electronic states and consequently, device performance. Factors such as local lattice distortion, point defects, and dopant position, cause the electronic states to fluctuate, and so disturb the quantitative understanding and precise control of quantum device characteristics. Therefore, not only the design and fabrication of a quantum device by QD arrangement, but also a structural analysis of each QD should be established for fundamental physics and industrial applications. X-ray absorption fine structure (XAFS) is a spectrum oscillation at an absorption edge. The sharp peaks at the edge originate in the electron transition from the inner-shell to valence states, indicating that the electronic states can be analyzed from the spectrum shape. On the other hand, the Fourier transform of the oscillation over the wide energy range provides radial atom distribution, i.e. bond lengths and coordination numbers [6,7]. In XAFS measurement, depending on the X-ray photon energy being tuned to the absorption edge of a specific atom, local information on the selected atom can be observed even in a compound (atom selectivity). However, in the case that the atom has different sites dependent on its chemical environments, the macroscopic X-ray absorption in conventional XAFS measurement provides only averaging information in the X-ray irradiation area. Despite the potential for local structure analysis by atom selectivity, the poor site-selectivity of XAFS analysis is obviously disadvantageous for nano-structure observation. In recent years, a new XAFS measurement technique, capacitance XAFS method [8,9], has been proposed. In this method, capacitance involved in semiconductor diode structure is measured under X-ray irradiation. Since the capacitance is sensitive to localized electrons, photoemission of the electron owing to the X-ray inner-shell absorption induces capacitance changes. Therefore, the X-ray photon energy dependence of the capacitance yields a site-selective XAFS spectrum of the atom with the localized electrons, such as defects [8,9], surfaces [10], and hetero-interfaces. In this study, the capacitance XAFS method is adopted for a self-organized InAs QD structure. Based on strain-induced mechanisms, it is natural to consider that the local distortion in the crystal lattice and/ or bond breaking in the self-organized QDs is introduced at the boundaries. However, this anomalous atomic structure has never been observed. According to the concept of the capacitance XAFS measurement, X-ray photoemission of confined electrons in the QD is expected to provide the selective XAFS spectra of the self-assembled atoms in each QD. 2. Selective QD observation by capacitance XAFS method Fig. 1 shows the proposed concept for the selective observation of QDs by the capacitance XAFS method. As indicated in Fig. 1(a), InAs QDs layers buried between the GaAs barrier layers are considered as a typical example. A sandwich structure formed by a top gate metal and a bottom conductive n-gaas substrate produces the equivalent of a capacitor. In this sense, the quantum level owing to the QD is considered to be the electron trap in a dielectric material. Therefore, the electronic property of the QD is evaluated in detail by conventional capacitance analyses, such as capacitance voltage (C V ) characteristics [11,12]. A band diagram of this structure and an expected electron

3 M. Ishii et al. / Microelectronic Engineering (2003) Fig. 1. Proposed concept for selective observation of QD by the capacitance XAFS method. (a) Typical sample structure and (b) expected electron transition process in the sample. transition process under the X-ray irradiation are schematically illustrated in Fig. 1(b). In this figure, the valence electronic states around the Fermi energy (E F) and inner-shell are simultaneously indicated. A bias voltage Vb applied to the sample causes an electric field in the capacitor, resulting in a band incline such that the EF of the gate metal corresponds to that of n-gaas. As shown in Fig. 1(b), at an appropriate V b, EF is aligned with the quantum level of an InAs QD, making it permissible for a resonant electron to tunnel from n-gaas to the QD (1). The X-ray absorption of this QD induces inner-shell excitation and makes a core-hole (2), then the confined electron in the QD relaxes in the core-hole (3). Since this sequential process from (1) to (3) is equivalent to X-ray photoemission of the confined electron, the number of confined electrons in the QD is determined by the amount of X-ray absorption. As a result, the X-ray absorption of the QD can be evaluated by the capacitance change under X-ray irradiation, and photon energy dependence on the capacitance gives the XAFS spectrum (capacitance XAFS method). Note that the capacitance XAFS method selectively observes the QD resonant with E ; the X-ray absorption of the specific QD, not the whole sample, can be obtained by F V selection [10]. b 3. Experiments and sample details The capacitance XAFS measurements were performed at a synchrotron radiation (SR) facility SPring-8 located in Hyogo Prefecture, Japan. The beamline was the BL10XU High Brilliance XAFS station [13], which has an in-vacuum-type undulator as its X-ray emitting source [14]. The SR beam was monochromatized by a Si (111) double crystal. A rhodium-coated double mirror was used to eliminate undesirable higher-order radiation such as third harmonics [15].

4 958 M. Ishii et al. / Microelectronic Engineering (2003) The InAs QD sample was fabricated by chemical beam epitaxy with trimetylindium, tryethylgallium, and precracked AsH3 as source materials [16]. In this sample, four InAs QD layers with a QD density of /cm were buried between GaAs barrier layers. The thickness of each barrier layer was fixed at 50 nm, while the QD size was estimated at 28 nm in diameter and 8nm in height by high-resolution scanning electron microscopy and atomic force microscopy. The aluminum gate metal was evaporated onto the 50-nm GaAs capping layer. The capacitance under the X-ray irradiation was measured by an impedance analyzer (590, Keithley). The frequency and amplitude of the AC power supply for the capacitance probe were 100 khz and 15 mv, respectively. The sample was mounted on a closed cycle He cryostat, and the temperature was fixed at 250 K. 4. Experimental results of selective observation of QD Fig. 2 shows the X-ray photon energy dependence of the capacitance involved in the self-organized InAs QD structure, i.e. capacitance XAFS spectra of QD. The Vb is varied from 0 to 0.8 V. These spectra all show an edge jump, followed by an absorption peak, indicating that X-ray absorption can be evaluated by the capacitance XAFS method. The absorption peak around the edge jump energy is generally known as the X-ray absorption near edge structure (XANES). The XANES spectrum originates from a resonant electron transition from the inner-shell to the unoccupied valence states. Since the unoccupied states strongly depend on the local structure of the atom, the XANES spectrum shape is discussed in detail in this paper. The edge jump in Fig. 2 corresponds to the As K-edge ( kev). Further careful observation of the absorption peak at kev reveals photon energy with the maximum X-ray absorption shifted at a specific V b; the peak photon energy at V b 0.2 and 0.6 V ( kev) is 1.5 ev lower Fig. 2. Capacitance XAFS spectra of self-organized InAs QDs. The inset indicates the C V characteristics of this sample.

5 M. Ishii et al. / Microelectronic Engineering (2003) than that of the other spectra ( kev). The resonant Vb dependence on spectrum shape indicates the site-selectivity of the capacitance XAFS method. The inset of Fig. 2 indicates the C V characteristics of this QD structure (open circles) and the derivatives of these C V characteristics dc/dv are also plotted in this figure (closed circles). The C V characteristics have a step-like structure, and the step voltage can be correctly estimated from the peak position of dc/dv, quite similar to results mentioned in previous reports [11,12]. As discussed in these reports, the capacitance step corresponds to the resonant electron tunneling to QD as denoted by process (1) in Fig. 1(b). From the peak position in dc/dv, it is understood that EF resonates with the QD level at V b 0.2 and 0.6 V. We note that these Vb values for the EF resonance are equal to Vb with the peak energy shift in the capacitance XAFS spectrum. This experimental finding indicates that the proposed model in Fig. 1(b) is reasonable; though the As atom is also included in the GaAs barrier layer, the capacitance XAFS measurement at Vb of 0.2 and 0.6 V provides a selective spectrum of the self-assembled As atom in the InAs QDs. The capacitance XAFS spectrum is observed at other V b s without the EF resonance because at the interface between n-gaas substrate and GaAs buffer layer, EF always corresponds with the dopant level independent of V b. With similar reference to Fig. 1, the X-ray induced photoemission of the electron from the dopant level yields the XAFS spectrum of the GaAs bulk around the interface [17]. Therefore, comparing the XAFS spectra with resonant Vb dependence and that without Vb dependence, the X-ray absorption peak intrinsic to As in a QD is distinguishable. In Fig. 2, all spectra except for V b 0.2 and 0.6 V indicate the X-ray absorption of As in the GaAs bulk. Considering the small difference between the chemical environment of As in the bulk InAs and that in GaAs, the peak energy shift of 1.5 ev at V of 0.2 and 0.6 V (Fig. 2) cannot be explained by a b chemical shift caused by the In atom. In fact, the conventional XAFS spectra of the bulk InAs and GaAs substrates make no difference to the peak photon energy. Hence, we conclude that the energy shift in Fig. 2 originates from a structural effect intrinsic to the self-organized InAs QD, rather than from a general material property of InAs. 5. Structure model of self-organized InAs QD X-ray absorption around the As K-edge is caused by the resonant electron transition from As 1s to 4p. To analyze the XANES spectra in more detail, the As 4p unoccupied states of InAs are evaluated by a first-principles molecular orbital (MO) calculation using discrete-variational (DV)-Xa method based on the self-consistent Hartree Fock Slater model [18]. In this calculation, two cluster models, In5As4 and In8As 7, are used. A cluster size dependence analysis of As 4p unoccupied states reveals the structural effect of InAs QD; while the large cluster approximately provides the InAs bulk property, the small cluster emphasizes the electronic states expected of the InAs QD. The solid line in Fig. 3 shows the unoccupied partial density of states (PDOS) of As 4p of (a) In As and (b) In As, with each cluster schematically illustrated in the inset of this figure. The atom coordination of these clusters is determined by the InAs zinc-blende structure. In this figure, 0 ev on the horizontal axis corresponds to the highest occupied molecular orbital (HOMO). As shown in this figure, the As 4p PDOS is distributed over the energy range of 0 9 ev. In the actual XAFS measurement, a short lifetime of 1s core-hole equivalently broadens these energy levels by the uncertainty principle, resulting in the single XANES peak with a broad bandwidth of 10 ev as

6 960 M. Ishii et al. / Microelectronic Engineering (2003) Fig. 3. Unoccupied states calculated by the first-principles molecular orbital calculation. shown in Fig. 2. Comparing Fig. 3(a) and (b), it is obvious that the structural effect is significant in the middle energy region of 5 ev. While the large cluster (a) has a PDOS at 4 ev, the small cluster (b) degenerates the PDOS into a single peak at 6 ev. The degeneracy in the small cluster isolates the PDOS at 0 ev from the other states in the higher energy region. In this situation, when the PDOS is broadened by the uncertainty principle, the peak position of XANES will mainly be determined by the PDOS at 0 ev. On the other hand, since the PDOS at 4 ev (Fig. 3(a)) smoothes the XANES spectrum curve, the bulk InAs is considered to have an absorption peak at higher energy. This consideration is consistent with the experimental result wherein the structural effect of the InAs QD is observed as the peak shift to the lower energy. The dotted line in Fig. 3 shows the In 5s PDOS. The In 5s PDOS of the small cluster (b) forms an obvious peak at 6 ev. Therefore, we recognize that the structure effect in this middle-energy region is closely related to the hybridization of As 4p and In 5s. The anti-bonding state of As 4p and In 5s is schematically shown in Fig. 4(a). In the InAs bulk (left figure), the hybridization is performed by the Fig. 4. The structural effect of InAs QD. Schematic view graphs of (a) orbital hybridization and (b) its energy diagram.

7 M. Ishii et al. / Microelectronic Engineering (2003) vertically polarized As 4p z. On the other hand, the reduction of the cluster size rotates a 4pz orbital to the x y plane (right figure), and yields degeneration of As 4px and 4p y. This degeneracy owing to the orbital rotation forms a s*-like state with a large orbital overlap. The energy eigenvalue of the anti-bonding state generally increases with increasing orbital overlap, therefore the s*-like state pushes up the energy level as shown in Fig. 4(b). The higher energy eigenvalue corresponds to the PDOS at 6 ev in the small cluster (Fig. 3(b)). In these calculations, we did not consider GaAs surrounding the InAs cluster. However, for the formation of s*-like states by the orbital rotation, the As 4p should be free from the atoms surrounding the cluster. Specifically, the structural effect discussed in Figs. 3 and 4 is based on a Ga As bond breaking, i.e. a point defect. Hence, the structural effect in the capacitance XAFS measurement is considered to indicate one piece of evidence of the point defect at the InAs QD boundary. 6. Conclusion For local structure analysis in self-organized InAs quantum dots (QD), a new experimental technique, capacitance X-ray absorption fine structure (XAFS) measurement, was adopted. In this method, the XAFS spectrum is measured by photon energy dependence of capacitance involved in diode structure, and the Fermi energy resonance with the quantum level by applied bias control is used for QD site selection. The X-ray-induced photoemission of confined electrons provides the site-selective XAFS spectrum of the QD in the capacitance change. An electronic state intrinsic to the InAs QD structure was found in the As K-edge absorption spectrum. A molecular orbital calculation indicates that this electronic state originates from a degeneracy of the As 4p orbital at the boundary of the InAs QD. Since the degeneracy is stimulated by the Ga As bond breaking, we strongly suggest that a point defect is introduced at the boundary between the InAs QD and the GaAs barrier layer. References [1] H. Sasaki, G. Yusa, T. Someya, Y. Ohno, T. Noda, H. Akiyama, Y. Kadoya, H. Noge, Appl. Phys. Lett. 67 (1995) [2] H. Shoji, K. Mukai, N. Ohtsuka, M. Sugawara, T. Uchida, H. Ishikawa, IEEE Photonics Technol. Lett. 7 (1995) [3] J.F. Carlin, R.P. Stanley, P. Pellandini, U. Oesterle, M. Ilegems, Appl. Phys. Lett. 75 (1999) 908. [4] J.M. Moison, F. Houzay, F. Barthe, L. Leprince, E. Andre, O. Vatel, Appl. Phys. Lett. 64 (1994) 196. [5] J. Oshinowo, M. Nishioka, S. Ishida, Y. Arakawa, Appl. Phys. Lett. 65 (1994) [6] F.W. Lytle, D.E. Sayers, E.A. Stern, Phys. Rev. B 11 (1975) [7] B.-K. Teo, P.A. Lee, J. Am. Chem. Soc. 101 (1979) [8] M. Ishii, Y. Yoshino, K. Takarabe, O. Shimomura, Appl. Phys. Lett. 74 (1999) [9] M. Ishii, Phys. Rev. B 65 (2002) [10] M. Ishii, Jpn. J. Appl. Phys. 41 (2002) [11] H. Drexler, D. Leonard, W. Hansen, J.P. Katthaus, P.M. Petriff, Phys. Rev. Lett. 73 (1994) [12] N. Horiguchi, T. Futatsugi, Y. Nakata, N. Yokoyama, Jpn. J. Appl. Phys. 36 (1997) L1246. [13] H. Oyanagi, M. Ishii, C.-H. Lee, N.L. Saini, Y. Kuwahara, A. Sato, Y. Izumi, H. Hashimono, J. Synchrotron Rad. 7 (2000) 89. [14] H. Kitamura, Rev. Sci. Instrum. 66 (1995) 2007.

8 962 M. Ishii et al. / Microelectronic Engineering (2003) [15] T. Uruga, H. Kimura, Y. Kohmura, M. Kuroda, H. Nagasawa, K. Ohtomo, H. Yamada, T. Ishikawa, T. Ueki, H. Iwasaki, S. Hashimoto, Y. Kashihara, K. Okui, Rev. Sci. Instrum. 66 (1995) [16] K. Ozasa, Y. Aoyagi, J. Cryst. Growth 188 (1998) 370. [17] M. Ishii, Jpn. J. Appl. Phys. 40 (2001) [18] H. Adachi, M. Tsukada, C. Satoko, J. Phys. Soc. Jpn. 45 (1978) 875.